1. Field of the Invention
The present invention relates to a recording material which is capable of yielding images having a high absorption intensity in a near infrared region, especially used as a thermosensitive recording sheet and a pressure-sensitive recording sheet.
2. Discussion of Background
Recording materials using leuco dyes are conventionally known, as shown in Japanese patent Publication No. 45-14039. These recording materials, which are utilized as pressure-sensitive recording sheets and thermosensitive recording sheets, are increasingly used year by year.
Leuco dyes in general use are triphenylmethane-type leuco dyes, fluoran-type leuco dyes, phenothiazine-type leuco dyes and auramine-type leuco dyes. These leuco dyes produce their own particular color tones and are selectively used depending on the application.
However, the above dyes have been developed, with an emphasis on the improvement of the color tone, that is, on the improvement of the absorption in the visible light region. In fact, dyes which can absorb near infrared rays having a wavelength of 700 to 1,000 nm have scarcely been developed.
As a semiconductor laser has become prevalent, a tendency to read recorded images such as bar codes by use of the semiconductor laser is growing. A demand for a thermosensitive recording sheet and a pressure-sensitive recording sheet which can exhibit a sufficient absorption intensity in a near infrared region for reading is also increasing. In particular, the spotlight of attention is focussed upon the wavelength range of the semiconductor laser beam from 650 to 800 nm. Accordingly, a demand for a pressure-sensitive recording sheet and a thermosensitive recording sheet capable of yielding images having a sufficient absorption intensity for reading within the above wavelength range is especially increasing.
A variety of the leuco dyes capable of absorbing light in a near infrared region and various thermosensitive recording materials and pressure-sensitive recording materials using the above leuco dyes are conventionally proposed. For example, phthalide compounds containing two vinyl groups are disclosed in Japanese Laid-Open patent application Nos. 51-121035, 58-157779, 60-27589 and 62-243653; and sulfonylmethane compounds are disclosed in Japanese Laid-Open patent application No. 60-231766. Furthermore, as the leuco dyes having a sufficient absorption intensity in the near infrared region of 650 to 800 nm, phthalide compounds having one vinyl group are disclosed in Japanese Laid-Open patent application Nos. 51-121037, 57-167979 and 63-165379.
However, the above-mentioned leuco dyes capable of absorbing near infrared rays and the recording materials using the same have their own shortcomings. For example, although the phthalide compounds containing two vinyl groups and sulfonylmethane compounds have an absorption intensity in the near infrared region up to about 1,000 nm, their absorption intensity in an infrared region from 700 to 800 nm is not sufficient, and the backgroud of the recording materials using such leuco dyes is yellowish in color.
The phthalide compounds containing one vinyl group, which have a sufficient absorption intensity in a near infrared region up to around 800 nm, have the shortcomings in that the image formation stability is poor, and the background is easily colored and turns yellow when exposed to the sunray and fluorescent lighting over a long period of time. In addition, most of the above phthalide compounds produce a blue to green color, so that a large quantities of a fluoran-type leuco dye which produces a black color must be added to the phthalide compounds for practical use to realize the image formation in black.